The invention relates to liquid cleaning systems of the type in which a liquid is pumped around the system by way of a self-cleaning, barrier filter of the so-called back-flush type, and in particular relates to the incorporation of a fluid-powered centrifugal cleaner to process the contaminated liquid back-flushed from the barrier filter.
Self-cleaning barrier filter units are well known in the art in which a housing contains a plurality of filter elements or discrete surface regions each having an inlet surface exposed to a common inlet chamber of the pumped fluid and an opposite, outlet, surface exposed to a common outlet chamber in which the filtrate, the cleaned fluid, is collected before leaving the unit still under pressure. In addition, the inlet chamber has a shield or cover member which is operable periodically to isolate each inlet surface in turn from the inlet chamber and connect it instead to a rejection conduit which leaves the housing, so that as the shield member moves from element to element the temporarily shielded element is subject to reverse flow from the outlet chamber which serves to lift contaminant debris from the inlet surface of the element and flush it along the rejection conduit.
Such self cleaning barrier filter units may operate in a so-called intermittent manner, in which the shield or cover member moves only occasionally between inlet surface regions, and possibly limiting flow through the inlet surface region for a predetermined interval less than the total time it serves that region and/or whilst it is changing between inlet surface regions, or so that the flow of back-flushed liquid is more continuous but from a continuously varying, and frequently cleaned, region of inlet surface.
Whereas it is conventional in full flow or barrier filter design that each filter unit passes liquid with minimum pressure difference across it (subject of course to having a pore size to trap particles of the required dimensions), it is found that to efficiently flush such trapped contaminant particles from a filter requires a large pressure difference to be available to establish a high flow rate through the filter and carry them from the inlet surface region. However, if the filter element should become significantly or completely blocked to liquid passage, possibly over a period of time by inefficient flushing of the contaminants, the prolonged and repeated application of significant pressure difference across the filter region risks effecting physical damage to the underlying structure of the pressure filter in that region.
Thus it is important to the operating life of the filter unit that the back-flushing removes contaminants from the back-flushed region efficiently and without permitting a progressive accumulation.
Most simply the back-flushed liquid, despite the high concentration of contaminants, is returned directly to the reservoir from where it is subsequently pumped around the system and the solid contaminants are again held by the barrier filter. Whereas such an arrangement ensures the contaminants are contained in the system between the reservoir and barrier filter, the liquid in the reservoir does become progressively more contaminated.
As indicated above, a conventional full-flow or barrier type filter element normally intends to produce a relatively small pressure drop across it as liquid flows therethrough and it is known to interpose such a further barrier filter between the back-flush arrangement and the reservoir to isolate solid contaminant without significantly reducing the pressure difference applied across the back-flushed region of the filter.
However, such further barrier filter whilst having of necessity a significant surface area, is capable only of trapping a relatively small volume of contaminants and therefore is necessarily coarse if it is not to become blocked in a very short time. In practice therefore, such filters serves only to trap large particles whilst permitting small particles of soot etc to remain in the liquid. Notwithstanding the efficacy of such a further filter or dependence upon pore size, it is still liable to require cleaning frequently to avoid introducing a significant pressure difference to the back-flush arrangement; to avoid closing down the system, which would negate any advantage from the back-flush arrangement in the barrier filter, it is considered a practical necessity to have at least two such further filters in a duplex arrangement so that one operates whilst the other is cleaned or replaced.
An example of such a barrier type filter having back flush arrangement is disclosed in EP-A1-136202.
It will be appreciated therefore that such form of filtering of back-flushed liquid involves capital and maintenance costs which may not be justified for all systems, particularly in view of the limited range of solid contaminants removed thereby.
Within a liquid circulation system including such a barrier type filter, it is known to complement the filter, which traps contaminant particles on the basis of size alone and has limited capability to hold such contaminants, with a centrifugal cleaner in which contaminant particles are separated on the basis of mass rather than size (although they are related for any particular substance) and which has the capacity to hold a significant volume of separated contaminants.
Typically, within a centrifugal cleaner a substantially vertically-mounted, high speed rotor includes a contaminant-depositing container, (more conveniently referred to simply as a contaminant container) through which the fluid is passed and in which solid contaminants are separated from the fluid to deposit on the container wall from which they can be periodically removed or the container replaced. Such a centrifugal cleaner may have its rotor driven by external coupling to an engine or like rotary plant with which used, which results in a complex and expensive arrangement, or may, as is more usual and considerably cheaper to implement, be driven by causing the fluid applied to the contaminant container under pressure to exit by way of tangentially directed nozzle means, the reaction to which spins the rotor at high speed essential for efficient centrifugal separation. Such a fluid-cleaner, in which the rotor is driven by the fluid being cleaned, is usually referred to as a self-powered centrifugal cleaner.
In such a self-powered centrifugal cleaner the liquid loses all energy in passing through the container, so that it has invariably been used in a by-pass mode, tapping liquid from a relatively high pressure part of the system in the vicinity of the full flow filter and returning it directly to the reservoir.
The use of such self-powered centrifugal cleaner in combination with a barrier type filter is disclosed in EP-A-0606578, the barrier type filter delivering a constant supply of liquid, some of which may be diverted through the centrifugal cleaner to by-pass the rest of the circulation system.
It is implicit in such self-powered centrifugal cleaners that reaction nozzle means must be dimensioned to create a significant pressure difference thereacross to rotate the contaminant container at a speed high enough to affect centrifugal separation and also that the pressure and rate of supply of the liquid and be adequate to effect continuous high speed rotation or separation will not be efficient.
As an alternative to self-powered centrifugal cleaning, wherein all of the fluid is both subjected to centrifugal cleaning within the contaminant chamber and creates rotation of the chamber by its ejection therefrom, it is known to split liquid supplied at uniformly high pressure into two streams, one to serving only to spin the contaminant chamber and the other to pass through for cleaning. Such arrangement is described in U.S. Pat. No. 3,791,576 and U.S. Pat. No. 3,784,092 which each discloses a centrifugal cleaner in which a liquid stream pumped from a reservoir at constant pressure is split so that part of it drives the centrifugal separation rotor and part of it entrains a dissimilar liquid, floating on the reservoir, into the separation rotor where the pumped and entrained liquids can be separated from the mixture along with any solid contaminants from the stream which are isolated and contained.
However, in relation to back-flushing liquid from a barrier type filter, consideration of employing such an alternative cleaning mechanism to a barrier-type filter becomes unattractive not only because of the potentially relatively low supply pressure and rate and consistency of liquid supply, after passing through two lower type filter regions in series, but more significantly because the pressure drop across the contaminant container, whether able to rotate or not, significantly limits that available across the barrier filter region for back-flushing to the extent that it may not clean completely and accumulation of contaminants occur. If complete blockage of the filter region were to occur, such that there is in effect no pressure drop across the centrifugal cleaner, when the filter region is subsequently exposed to back-flushing pressure, the aforementioned high and potentially damaging pressure difference across the region could occur.
Therefore cleaning back-flushed contaminated liquid by way of conventially employed barrier-type or self-powered centrifugal cleaning devices has not been perceived as an efficient cost effective activity.
It is an object of the present invention to provide a liquid cleaning system, including pump means and a barrier type filter having an intermittent back-flush arrangement, in which the heavily contaminated back-flushed liquid is cleaned efficiently and simply by centrifugal cleaning.
According to a first aspect of the present invention, a liquid cleaning system comprises pump means, operable to cause liquid to flow through the system, a barrier type filter having a back-flush arrangement whereby a proportion of the liquid after passage through the filter barrier can be passed in a reverse direction through a region of said barrier, and a fluid-powered centrifugal cleaner including a rotor having a contaminant chamber through which the back-flushed liquid is passed and rotor drive means responsive to consistently supplied fluid, separate from the back-flushed liquid, to effect rotation of the rotor about a substantially vertical axis at at least a minimum speed required to effect centrifugal separation of solid contaminants from back-flushed liquid passing through the contaminant chamber.
An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings.